There is one major industry which has been predicted and anticipated for many years, but has yet to materialize: the production and sale of small, affordable, vertical take-off and landing (VTOL) aircraft, called a “Personal Aircraft” or “PAC” for short. The present invention relates to the fundamental characteristics of this craft.
Since the appearance of the ubiquitous automobile, people have simply assumed that an inexpensive, easy to fly, VTOL aircraft would someday be in our future. As early as 1926, no less a visionary and hard-nosed businessman than Henry Ford himself attempted to develop the “Ford flying flivver” but the technology of the day was simply not up to the task.
Initially it was thought that the helicopter, developed by Igor Sikorsky, would become the personal mode of transportation envisioned by Henry Ford, but for several reasons it has remained a special-purpose aircraft. Principally due to its control system, which includes a complex mechanism for continuously adjusting the pitch of individual rotating blades, the helicopter is extremely expensive to construct and to maintain. It is also not possible to provide redundancy so that failure of any drive element can result in a crash. Further, the helicopter is notoriously difficult to fly.
Prof. Paul Moller has for many years been developing what he calls a “SkyCar”: a two or four-place PAC which utilizes “ducted fans” for both lift and horizontal flight. Using four Wankel engines developing 650 horsepower on take-off and about 200 Hp for level flight, the craft is designed for a cruising speed of 300 MPH.
Although the SkyCar may have certain advantages over a helicopter, it requires all four of its Wankel engines to operate properly at all times to remain airworthy. Another disadvantage is its relatively high rate of fuel consumption, both while hovering and in forward, horizontal flight, as compared to a helicopter. Its major problem is that, in replacing a helicopter's “rotary wings” with small ducted fans, it requires approximately four times the horsepower to maintain it aloft, in the hover mode.
As this term is used herein, a “ducted fan” is simply a device for accelerating air as it passes through a duct or shroud. The duct serves primarily to limit radial flow outwards from the hub to the blade tip, thereby increasing the axial flow of air through the duct and increasing the “thrust”, which is the force of reaction to the acceleration of air, as compared to a non-ducted fan or propeller. The air is accelerated through the duct by a multi-bladed fan or “impeller” and the swirl, or rotational component if the flow, is usually converted back to axial flow by a further row of blades which are stationery and hence referred to as the “stator”. Alternatively, the rotational component may be converted back to axial flow by a second impeller rotating n the opposite direction from the first impeller.
The static thrust T (in pounds) of a ducted fan unit with a single impeller is given approximately by the formula:T=13(E×Hp×d)2/3,  (1)
where E is the efficiency of the ducted fan (anywhere from 0 to 1 for 0% to 100%), Hp is the horsepower of the engine driving the impeller (in bhp), and d is the ducted fan outlet diameter (in feet).
If the impeller efficiency is assumed to be ⅔ (0.67), and the fan outlet diameter 1.5 feet, then E×d=1, and the formula reduces to:T=13 Hp2/3.To lift 2000 pounds with four ducted fan units, as does the Moller SkyCar, each unit must develop at least 500 pounds of thrust. Setting T=500,Hp2/3=500/13=38.46,Hp=168.6 bhp.
Therefore a fully loaded 2000 pound SkyCar provided with four ducted fan units can lift off vertically and hover if each ducted fan is driven with an engine developing 168.6 brake horsepower. The total horsepower requirement is therefore approximately 168.6×4 or 675 Hp. This is about four times the engine horsepower requirement for a light (two-place) helicopter. In addition to the expense and weight of the engines, the fuel burn rate required to develop for such horsepower is extremely high, as compared to a helicopter, and gives rise to the need for a more efficient design.